The invention relates to a process for suppressing the increase in specific average pore diameter which occurs when a silica is subjected to a hydrothermal treatment.
Porous silica is used widely, inter alia, as a drying agent, a catalyst, a catalyst carrier and as an adsorbent in chromatographic separations. In each of these applications the specific average pore diameter will, for the sake of brevity, be indicated by the letter p. For further information about p and the way in which it is determined reference is made to Netherlands patent application No. 7214397 or U.S. Pat. No. 3,925,197 wherein in column 3 this catalyst characteristic is discussed in detail.
It is very important that once a silica has the proper p for a certain application, this p should change as little as possible during all further treatments to which the silica is subjected. However, sometimes the problem arises that p of a silica increases when it is subjected to a hydrothermal treatment, by which is understood that the silica is treated at elevated temperature with liquid water and/or water vapor.
For reasons hitherto unknown, with a number of silicas the hydrothermal treatment has little or no effect on p. It has further been found that under certain conditions of temperature and pressure a hydrothermal treatment may cause a much stronger increase in p with one silica than with another. Generally, when a silica shows a distinct increase in p upon hydrothermal treatment, this increase is larger as the treatment is effected under more severe conditions, i.e. at a higher temperature and/or pressure.
For the application of silica as catalysts and catalyst supports, particularly for hydrocarbon treating processes such as hydrodesulphurization and demetallization of residual oils, as well as in the regeneration of such catalysts, the problem concerning the increase in p of silicas under hydrothermal conditions has been troublesome. It was decided to carry out an extensive investigation to provide more insight into this matter. In this investigation it was found that the increase in p, which occurs when silicas are subjected to a hydrothermal treatment, can be greatly suppressed by carrying out said hydrothermal treatment in the presence of an added quantity of at least one compound of any of the elements magnesium, iron, aluminum and titanium.
This finding is thought to be very important, because it greatly enhances the applicability of silicas and in principle enables silicas to be exposed to hydrothermal conditions without the risk of too great an increase in p. This finding is the subject of the present patent application.